1. Field of the Invention
The invention relates to a main spindle device for a machine tool, and in particular, to an air sealing structure that forms a gap between a main spindle and a housing.
2. Description of the Related Art
A main spindle used in a main spindle device of a machine tool is inserted into a housing, and supported by a bearing so as to be able to rotate relative to the housing. A tool, a workpiece, or the like is held at the tip of the main spindle to perform machining, and in general, machining is performed while spraying coolant etc. (cutting fluid etc.) onto the tool, the workpiece, or the like, for the purpose of preventing seizure at the machined portion, improving the plane accuracy, etc. However, coolant, foreign matter (chips), etc. can easily pass through the gap between the housing and the main spindle and reach the bearing. When the coolant etc. reach the bearing, there is a possibility that seizure of the bearing occurs. For this reason, various types of sealing structures have been used to prevent the coolant, foreign matter, etc. from entering through the gap between the housing and the main spindle and reaching the bearing, and in general, air sealing is used.
For example, the related art described in Japanese Patent Application Publication No. 2000-18395 (JP-A-2000-18395) is a main spindle device 101 that uses an air sealing structure shown in FIGS. 4A, 4B and 5 and includes a non-contact, air sealing structure that effects sealing by discharging compressed air (hereinafter referred to as “air”) through a gap between a housing, which includes a housing body 20 and a bearing holding lid member 130, and a main spindle 110. FIG. 4A shows a sectional view taken along the plane including the rotation axis ZT of the main spindle 110 (note that a tool T and a tool holder H are not shown in section). FIG. 4B shows a diagram when viewed from the direction indicated by the arrow BB with the tool T and the tool holder H omitted from FIG. 4A. FIG. 5 is an enlarged view of the AS1 portion in FIG. 4A. As shown in FIG. 4A, the tool T is fitted to the tool holder H, which is in turn fitted to a tip portion of the main spindle 110. The main spindle 110 is inserted into the housing, which includes the housing body 20 and the bearing holding lid member 130, and supported by a bearing J so as to be able to rotate about the rotation axis ZT relative to the housing body 20.
The bearing holding lid member 130 in which a hole is formed, into which the main spindle 110 can be inserted, is fitted into the housing body 20 along the direction of the rotation axis ZT, and the air sealing structure is formed between the inner circumferential surface of the bearing holding lid member 130 and the outer circumferential surface of the main spindle 110. As shown in FIG. 5, which is an enlarged view of the AS1 portion in FIG. 4A, a first annular groove M1, a second annular groove M2, and a third annular groove M3 are formed on an outer circumferential surface of the main spindle 110 in the circumferential direction. On part of an inner circumferential surface of the bearing holding lid member 130 that faces the first annular groove M1, an inner-side air receiving groove MA is formed in the circumferential direction. On part of the inner circumferential surface of the bearing holding lid member 130 that faces the second annular groove M2, an inner-side air receiving groove MB is formed in the circumferential direction. On part of the inner circumferential surface of the bearing holding lid member 130 that faces the third annular groove M3, an inner-side air receiving groove MC is formed in the circumferential direction. An air receiving area AK1 is created by the first annular groove M1 and the inner-side air receiving groove MA, a collection space AK2 is created by the second annular groove M2 and the inner-side air receiving groove MB, and an air receiving area AK3 is created by the third annular groove M3 and the inner-side air receiving groove MC. The air supplied from the air supply source (not shown) is distributed to air supply passages AL1 and AL2 and supplied to the first annular groove M1 (air receiving area AK1) and the third annular groove M3 (air receiving area AK3). The air supplied to the air receiving area AK1 is discharged through air sealing gaps AG1 and AG2 to effect air sealing, and the air supplied to the air receiving area AK3 is discharged through air sealing gaps AG3 and AG4 to effect air sealing. In case that coolant etc. pass through the air sealing gaps AG1 and AG2, a discharging passage DL for discharging the entered coolant etc. is formed in a lower portion of the inner circumferential surface of the housing that faces the collection space AK2, whereby the entrance of coolant etc. into the bearing J is prevented.
In the case of the related art described in JP-A-2000-18395, the discharge of air from the first annular groove M1, to which the air supply passage AL1 is connected, and the discharge of air from the third annular groove M3, to which the air supply passage AL2 is connected, do not occur uniformly all around the periphery of the main spindle 110, that is, air is discharged mostly at the portions at which the air supply passages AL1 and AL2 are connected, and the amount of air discharged decreases with the distance from such connection portions. This is because air is discharged through the air sealing gaps AG1, AG2, AG3, and AG4 before the air receiving areas AK1 and AK3 shown in FIG. 5 are sufficiently filled with air. Thus, coolant, foreign matter, etc. enter through the portions on the outer circumferential surface of the main spindle 110, at which the amount of air discharged is relatively small, which can cause seizure of the bearing J. The air sealing gaps AG1 to AG4 are set to approximately 50 μm, for example, to provide air sealing function, and are therefore very small gaps. Thus, when the tool T, the tool holder H, and/or the main spindle 110 interfere (collide) with another member due to an operational error by the operator, a defect in the machining program, etc., in some cases, the main spindle 110 contacts the housing (bearing holding lid member 130) and damage and/or seizure can occur. In general, a selection is made so that the material for the bearing holding lid member 130 has a mechanical strength lower than that of the main spindle 110, and when the damage of the bearing holding lid member 130 due to the collision is severe, the bearing holding lid member 130 has to be replaced. However, much time and costs are required to produce a bearing holding lid member 130 that is a relatively large part, and to replace the damaged bearing holding lid member 130.